1. Field of the Invention
The embodiments of the invention relate to a display device, and more particularly, to an organic light emitting diode display device and a driving method thereof. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for compensating the variation in threshold voltage of a thin film transistor caused by gate bias stress.
2. Discussion of the Related Art
Recently, flat display panels with reduced weight and size have been developed to replace the cathode ray tube display device, which is heavy and bulky. Such flat panel display devices include a liquid crystal display (hereinafter, referred to as “LCD”) device, a field emission display (hereinafter, referred to as “FED”) device, a plasma display panel (hereinafter, referred to as “PDP”) device, and an electro-luminescence (hereinafter, referred to as “EL”) display device. A PDP has light weight, thin profile, simple structure and is easy to manufacture. However, a PDP has low light-emission efficiency and requires large power consumption. The LCD device employs a thin film transistor (“TFT”) as a switching device. The TFT LCD device has the problems of narrow viewing angle and low response speed due to the required need of switching liquid crystal molecules to control light from a backlight. An EL display device is generally classified as either an inorganic EL display device or an organic light-emitting diode display device depending upon the material of a light-emitting layer. An EL display device is self-luminous. When compared with the above-mentioned display devices, the EL device generally has faster response speed, higher light-emission efficiency, greater brightness and wider viewing angle.
An organic light emitting diode device, as illustrated in FIG. 1, includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL formed between an anode electrode and a cathode electrode. When a driving voltage is applied across the anode electrode and the cathode electrode, a hole within the hole injection layer and an electron within the electron injection layer respectively move toward the emission layer EML to form excitons. As a result of exciton generation, the emission layer EML emits visible rays.
A plurality of the organic light emitting diode display devices shown in FIG. 1 can be arranged in a matrix array. By selecting each of the organic light emitting diode devices with a scan voltage for driving by a data voltage, the brightness of the matrix array of organic light emitting diode devices can be controlled in accordance with digital video data. The above-described matrix array of organic light-emitting diode display devices is classified as either a passive matrix type display device or an active matrix type display, which uses a TFT as a switching element. In the active matrix type, an organic light emitting diode device is selected by using a TFT, which is an active device, such that the organic light emitting diode device is driven.
FIG. 2 is a circuit diagram equivalently showing one pixel in an organic light-emitting diode display device of an active matrix type. As shown in FIG. 2, the organic light-emitting diode display device of the active matrix type includes an organic light-emitting diode element OLED, a data line DL and a gate line GL that cross each other, a switch TFT ST, a driving TFT DRT and a storage capacitor. The driving TFT ST and the switch TFT DRT are implemented as N-type metal-oxide-semiconductor field-effect transistors (“MOS-FETs’).
The switch TFT SWT turns on in response to scan pulses from the gate line GL to electrically connect a current path between a source electrode and a drain electrode of the switch TFT SWT. A positive data voltage from the data line DL is applied, via the source electrode and the drain electrode of the switch TFT SWT, to a gate electrode and a storage capacitor of the driving TFT DRT during an on-time period of the switch TFT SWT. The driving TFT DRT supplies current to the organic light emitting diode OLED in accordance with a gate voltage supplied to its gate electrode, i.e., a positive data voltage, to drive the organic light emitting diode OLED. The storage capacitor stores a difference voltage between the positive data voltage and a low-level power supply voltage VSS, which constantly maintains a voltage applied to the gate electrode of the driving TFT DRT during one frame period. The organic light-emitting diode display OLED has the structure shown in FIG. 1.
The brightness of a cell, as shown in FIG. 2, is proportional to current flowing through the organic light emitting diode OLED, and the current is adjusted by a gate voltage of the driving TFT DRT. The current IOLED of the organic light emitting diode OLED flowing through the driving TFT DRT is defined by the following Equation (1).
                                                                        I                OLED                            =                            ⁢                                                β                  2                                ⁢                                                      (                                          Vgs                      -                                                                      Vth                                                                                      )                                    2                                                                                                        =                            ⁢                                                k                  2                                ⁢                                  W                  L                                ⁢                                                      (                                          Vdata                      -                      VSS                      -                                                                      Vth                                                                                      )                                    2                                                                                        (        1        )            wherein, ‘Vth’ represents a threshold voltage of the driving TFT DRT, ‘k’ represents a constant defined by mobility and a parasitic capacitance of the driving TFT DRT, ‘L’ represents a channel length of the driving TFT DRT and ‘W’ represents a channel width of the driving TFT DRT, respectively.
As shown in Equation 1, the current IOLED of the organic light emitting diode OLED varies in accordance with the threshold voltage Vth or mobility of the driving TFT DRT. Therefore, to ensure uniform picture quality of displayed images on the organic light emitting diode display device, all of the driving TFTs DRT of the entire display device are required to have uniform electrical characteristics. However, the threshold voltage Vth of the driving TFT DRT varies due to a gate bias stress, and as a result, degradation of current flowing through the organic light emitting diode OLED increases with time, thereby lowering the reliability of driving.
FIGS. 3 and 4 are graphs showing examples of variations in the threshold voltage of a thin film transistor caused by gate bias stress. Gate bias stress is the phenomenon in which the threshold voltage Vth of a TFT is shifted when the gate voltage of the TFT is a continuously applied positive voltage (positive gate bias stress), as illustrated in FIG. 3, or a continuously applied negative voltage (negative gate bias stress), as illustrated in FIG. 4. In FIGS. 3 and 4, the horizontal axis represents the gate voltage Vg applied to the gate electrode of the TFT, while the longitudinal axis represents drain-source current Ids of the TFT. The threshold voltage of the TFT becomes higher due to the positive gate bias stress of FIG. 3, and the threshold voltage of the TFT becomes lower due to the negative gate bias stress of FIG. 4. Such a gate bias stress is caused by charge trapping in which ions are caught in the insulating layer between the electrodes of the TFT. The trapping creates a defect in that the threshold voltage of the TFT shifts such that mobility in the channel layer is changed.
To compensate for threshold voltage shift of the driving TFT DRT in each of circuits of light emitting cells of the organic light emitting diode display device as illustrated in FIG. 2, a BDI (black data insertion) driving method has been proposed in which both a negative compensation voltage and a positive data voltage are applied to the gate electrode of the driving TFT DRT within 1 frame period.
FIG. 5 shows a driving waveform of the related art BDI driving method. As shown in FIG. 5, in the BDI driving method, 1 frame period is time-divided into an emission period (on) and a non-emission period (off) for driving the light emitting cells. In the related art BDI driving method, as shown in FIG. 5, as the emission-on period ends with an emission-off period within 1 frame period, the data voltage is turned off so the effect of reduction of gate bias stress is low when a negative compensation voltage is applied.